I actually happen to have all 3. And honestly I would say it depends on what you wAnt. All 3 are very different devices and honestly if you are having to choose one machine and want to do a lot of song sketching qy70 hands down.
But if you are going to explore sound design the ds10 is much more capable with its sequencer and automation over he mono. But if you like hands on improv then mono. If u get all 3 and the amazing machines midi it clock syncs on the mono sync in and can sync the qy if u hard pan a ds10 drum for the sync signal. So you all sorts of fun in theory.. I'm working on option 3 myself.